Anatomy of a fumarole field; drone remote sensing and petrological approaches reveal the degassing and alteration structure at La Fossa cone, Vulcano Island, Italy

Müller, Daniel; Walter, Thomas R.; Troll, Valentin R.; Stammeier, Jessica; Karlsson, Andreas; De Paolo, Erica; Pisciotta, Antonino Fabio; Zimmer, Martin; De Jarnatt, Benjamin

doi:https://doi.org/10.5194/egusphere-2023-1692

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https://doi.org/10.5194/egusphere-2023-1692

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28 Jul 2023

| 28 Jul 2023

Anatomy of a fumarole field; drone remote sensing and petrological approaches reveal the degassing and alteration structure at La Fossa cone, Vulcano Island, Italy

Daniel Müller, Thomas R. Walter, Valentin R. Troll, Jessica Stammeier, Andreas Karlsson, Erica De Paolo, Antonino Fabio Pisciotta, Martin Zimmer, and Benjamin De Jarnatt

Abstract. Hydrothermal alteration processes can affect the physical and chemical properties of volcanic rocks and develop via complex degassing and fluid flow systems and regimes. Although alteration can have far-reaching consequences for rock stability and permeability, little is known about the detailed structures, extent, and dynamic changes that take place in hydrothermal venting systems. By combining drone-based remote sensing with mineralogical and chemical analyses of rock and gas samples, we analyzed the structure and internal anatomy of a dynamic evolving volcanic degassing and alteration system at the La Fossa cone, Vulcano Island (Italy). From drone image analysis, we revealed a ~70,000 m² sized area subject to hydrothermal activity, for which we could determine distinct alteration gradients. By mineralogical and geochemical sampling of the zones of those alteration gradients, we study the relation between surface coloration and mineralogical and chemical composition. With increasing pixel brightness towards higher alteration gradients, we find a loss of initial mineral fraction and bulk chemical composition and a simultaneous gain in sulfur content. Using this approach, we defined and spatially constrained alteration units and compared them to the present-day thermally active surface and degassing pattern. The combined results permit us to present a detailed anatomy of the La Fossa fumarole field, highlighting 7 major units of alteration and present-day diffuse activity that, next to the high-temperature fumaroles, significantly contribute to the total activity.

Received: 23 Jul 2023 – Discussion started: 28 Jul 2023

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Daniel Müller, Thomas R. Walter, Valentin R. Troll, Jessica Stammeier, Andreas Karlsson, Erica De Paolo, Antonino Fabio Pisciotta, Martin Zimmer, and Benjamin De Jarnatt

Status: final response (author comments only)

RC1:
'Comment on egusphere-2023-1692', David Jessop, 30 Oct 2023
Dear Editor,
Please find below my report on the manuscript “Anatomy of a fumarole field; drone remote sensing and petrological approaches reveal the degassing and alteration structure at La Fossa cone, Vulcano Island, Italy” (egusphere-2023-1692) by Daniel Müller et al.
The present manuscript presents a combination of visible and thermal imagery, mineral and geochemical analyses of rock samples and CO2 soil degassing data with the aim of classifying and quantifying the alteration and degassing structures at the Vulcano Fossa volcano. The manuscript uses a number of novel techniques, with the main emphasis on the classification of thermal and visible images (orthophotos), similar to previously published work (Müller et al., 2021), with the addition of mineralogical and geochemical analyses of hydrothermally altered samples. Whilst this approach is promising, and the results given are interesting, I think that some of the methods employed need much more extensive descriptions. Some of the techniques employed may also be flawed or, at best, misunderstood. Lastly, I do not feel that the authorship is currently up to standard for publication, though the written English is fine in itself. Hence it is my opinion that the manuscript requires major revisions before it could be considered for publication. I provide some suggestions that the authors may chose to follow. Given that the revisions may be substatial, a review of the revised manuscript may be necessary. In this case I would be happy to act as referee should you require my services.

Best regards

David Jessop

PCA and image classification
Probably my major concern regarding the manuscript concerns the lack of description of the “Principal Component Analysis”, (PCA) and the image classifications. My reasons are two fold:

The PCA and image classification processes, provided within the ArcGIS propitiatory software, are “black boxes” with no description of what is happening under the hood. As written, it is not easy to understand how these process work, or what data they provide. The interested (and reasonably competent) reader should be able to reproduce the results of this study and I don’t think that is currently the case.

In order to reproduce these results, the interested reader would require access to the proprietary ArgGIS software which means paying 500-700€. I don’t think that this is fair. Whilst this may not be an issue for wealthy universities, it could block researchers with less access to funding from utilising the approaches employed in the present study. A proper description of the methods could allow such researchers to look for cheaper of even open-source alternatives.

PCA consists of taking a multi-dimensional dataset and finding the orthonormal basis vector space that describes the data whilst minimising the variance along each vector basis (component). This is achieved by calculating by projection of the data onto a set of orthogonal axes where the variance of each data set is represented by the eigenvalues of the data. These eigenvalues are the principal components. This is usually achieved using a reduced singular value decomposition (SVD) which produces the eigenvalues of the dataset. PCA then takes the list of ordered eigenvalues which are typically used to perform dimensional reduction in high-dimensional data sets. This consists of rejecting any components that do not contribute significantly to the overall variance of the data. In the present study this is applied to an RGB image (i.e. 3D) and the authors take 3 principal components, so there is no dimensional reduction and hence the “PCA” is kind of redundant.
Regardless of how we name this process, it is unclear what the ArcGIS algorithm produces in the “Principal Component” band images (cf. L220-223) – PCA gives only the variance. Please indicate how this information is used to transform the RGB image.
The image classification, named as “unsupervised classification”, process is poorly documented. Indeed unsupervised classification is a blanket term for a multitude of different families of algorithms so, to be able to reproduce these results, one would have to know which algorithm was chosen and why. The choice of 32 classes seems to be completely arbitrary (50 were used in Müller et al., 2021) and, furthermore, the individual classes are then regrouped (see fig. A1 for example). Could a smaller set of classes (say 4) not have been used to obtain similar results? I strongly urge the authors to justify their choice.
Whilst the authors refer to previous work (Müller et al., 2021) as a source for their methods, but neither the PCA nor the classification strategies are sufficiently well described in that work either. Without having to detail the algorithms in their entirety, the authors should please sufficiently explain their methods in the present manuscript so that they can be followed with the aim of reproducing their results.

Thermal image processing
F ig. 1: “Brightness” temperature predicted by Planck’s law compared to the scaling proposed by the authors (eq. 1). The two curves cross at a temperature of 383 K (about 90°C).

The authors state that they produce temperature maps from the 16-bit radiometric greyscale orthophoto using a linear mapping given by their eq. 1 using the radiometric resolution as a scaling factor. Owing to the non-linear behaviour of IR sensors this would seem unlikely to hold for more than a very limited range of greyscale values. Furthermore, it is typically necessary to use Planck’s law which predicts the “brightness” temperature of an object from the intensity of incoming radiation registered by the sensor (i.e. radiometric value). By way of illustration, I have produced the above graphic (Fig. 1). Here we see that there is only one point of intersection for the two curves and the predicted temperatures can be drastically different. That said, I am not familiar with the FLIR Tau camera as used in this study and do not have access to the radiometric conversion factors necessary to correctly plot the curve for this camera. If it is like many other FLIR and other IR cameras that I have used, this information can be found in the EXIF (image metadata) which can be readily extracted using the ExifTool software, for example. However, the authors should check their data and any calculations that depend on the temperature. Some of the stated temperatures and thresholds (e.g. 40 °C for the “high-temperature fumaroles”) are rather low given the vent temperatures recorded in other works (vent temperatures are well in excess of 100°C, cf. Diliberto, 2017; Mannini et al., 2019; Diliberto, 2021).

Authorship and increasing the scope of the paper
A quick survey of the the first dozen or so references showed that they were cited once in the introduction and nowhere else in the manuscript. The discussion contains only six references, and is often a rehash of the results section rather than a forum for putting these results into a fuller context and comparing them to previous works on Vulcano and, potentially, other volcanoes. Several references (e.g. Chiodini et al., 1996; Chiodini et al., 2005; Harris et al., 2009; Mannini et al., 2019) have made estimations of degassing and/or heat budgets with Harris et al. (2009), in particular, having made detailed descriptions of the fumarole field. I find it strange that the authors have not chosen to make the comparison with these works, particularly given the ongoing and unrest at Vulcano with the recent well-documented paroxysms. Fig. 11 identifies a structure with an increased radiant density and labels it as “new fumarole complex in 2021”, but this is not discussed anywhere. This would be very important information for assessing the activity at this volcano. Curiously, Fig. 10 hints at a heat budget having being calculated, but this is not discussed in the manuscript. I note also that the area of the alteration zone (ALTZ), that is the area affected by hydrothermal activity, is given as 70 000 m² (note typo “770 000 m²” on L301) which is very close to the “diffuse heated area” of 63 000 m² calculated by Mannini et al. (2019) using approximately contemporaneous data. Of course, I have my own professional biases in mentioning this, but the authors have already done the work so it is suprising that it is only mentioned in passing.
Concerning alteration of the edifice, there have been a number of studies in recent years trying to ascribe thermal properties of volcanic rocks to hydrothermally altered samples (typically andesite). These results may be interesting to discuss. See Heap et al., (2022) in particular. Section 5.3 briefly mentions the role of alteration and permeability:
“Relative gas flux values measured within unit c are lower than observed for units a and b, for instance. This might be a consequence of the dynamics of hydrothermal alteration and indicate permeability reduction or sealing processes due to the advanced state of alteration like proposed by Heap et al., 2019.” (L592-594)
Furthermore, a sequence of alternating high and low permeability zones are identified in Fig. 11, but each result is only discussed independently and there is no real synthesis of the large and important set of results. This is one discussion point, in particular, that is really important for assessing volcanic hazard and I find it frustrating that this point has not been fully persued.
A minima, the figure captions should allow the reader to understand the figure in isolation and, currently, this is not the case for several of the figures. The main cuprits are
Fig 2 – give locations of each photo, also show these locations on Fig 1B.
3 – describe the grey blocks. Why are there two blocks for Remote Sensing?
8 – what are “RGB values”, as only one value is given here. Is this for one band in particular or an average? Why are RGB values being used rather than one of the “PCA” image bands?
9 – What is the absicssa in this figure? How is it that Transect A blends into Transect B? I think it would be worthwhile to combine this figure with Fig. 7, particularly as one requires the locations of the transects to understand what is going on in this figure.
Other suggestions for the figure captions are:
4 – D, please give the thermal thresholds for each class.
5 – Please label your axes. The titles “Red channel values” should probably be axes labels.
6 – The bars are very confusing and there is no scale given for the gas concentrations. Instead maybe use a colour map where colour/intensity corresponds to value? What do the black dots mean? What is the direction of measurement for the “Distance from ALTZ boundary”, i.e. are positive values inside and distance is taken normal to some boundary? If so, which and how determined?
7 – (and elsewhere) it would be useful to have the definitions of ALT, AMT, LTZ, XRD, XRF etc. recalled here. Capitalise XRF (L694)
10 – Please state (either here or in the main text) how Rcum is calculated. Please also use proper representations of units in your axes labels (e.g. “W/sqm”)
11 – Please label the abscissa in D and give units. Please also use proper units for ordonate label (“kW/sqm”).
Generally, there is no description of the figures in the text of the style “In Figure X we show…”, rather the figures are referred to en passant (e.g. “pixels of Type 1 to 4 surface show a general increase of mean pixel temperatures from Type 4 to Type 1 surface by an average of 2 degrees (Figure 5).”, L330-332). This may be a deliberate stylistic choice by the authors but it this makes it harder still to understand the figures, and leads to the impression that they are not very important, particularly given the paucity of the descriptions in the captions.
Citation: https://doi.org/10.5194/egusphere-2023-1692-RC1
- AC1:
  'Reply on RC1', Daniel Müller, 24 May 2024
  Dear Reviewer 1
  We thank you for reviewing our manuscript. Your review was highly appreciated and helpful. We further want to apologize that our reply reached you so late. However, it took very long until a second review was provided, which is why our response reaches you now after an extended period.
  We carefully read through your suggestions and critiques and found them very helpful. We addressed all the points mentioned. We have changed the manuscript considerably and hope that it will now meet with your approval.
  In detail, we have modified:
  figure captions, which now better explain all necessary information shown in the figure
  
  the authorship by including publications and results of earlier works as suggested by you
  
  the discussion, now referring to results of earlier works and better highlighting how we contribute but also better synthesizing our results.
  
  We hope our replies are accepted and modifications made improve the manuscript. A more detailed reply to your revision is provided in the attached Reply letter. Some parts of the statements were complex and made up of several arguments. In such case, we split your original statements and replied to single arguments directly. You will find your original review text in black and our responses as Reply in blue.
  Thank you very much for supporting our work with your review and expertise, especially for motivating us to better link and compare results with earlier original studies done by other authors.
  Best Regards
  Daniel Müller
  
  Citation: https://doi.org/10.5194/egusphere-2023-1692-AC1
RC2:
'Comment on egusphere-2023-1692', Giancarlo Tamburello, 01 Apr 2024

The paper presents a comprehensive and meticulous investigation into the degassing and alteration structures of the fumarole field at La Fossa cone on Vulcano, offering valuable insights into the complex dynamics of volcanic activity. Using innovative methodologies, including close-range remote sensing, mineralogical and geochemical analyses, and surface degassing measurements, the authors provide a detailed and multi-faceted examination of the degassing system.
One of the study's most commendable aspects is its integration of high-resolution drone-derived imagery with traditional analytical techniques. This approach enables a nuanced spatial analysis, allowing the authors to accurately identify and characterize major active units. Furthermore, the quantification of thermal energy release provides valuable quantitative data on the relative importance of different degassing features within the system, enhancing our understanding of volcanic processes.
The authors demonstrate a commendable level of transparency and rigour by acknowledging potential limitations, such as gas plume distortion affecting image quality. This ensures the reliability and validity of their findings, reflecting a commitment to scientific integrity and strengthening the credibility of the study's conclusions.
Overall, this paper represents a significant contribution to the field of volcanic degassing research. By elucidating the complex interplay between surface manifestations, alteration gradients, and gas emissions, the study advances our understanding of volcanic systems and provides a solid foundation for future research in this area. Its comprehensive approach and meticulous attention to detail make it a valuable resource for scientists and researchers working in volcano monitoring and hazard assessment.

Here I list minor comments that I hope may improve the final version of the manuscript:
- The authors mention some fumaroles (e.g., F0) but do not show them in the figures in the main text (only supplementary). They are not also mentioned in the discussion. The fumaroles F0, FA, F5AT and F11 have distinct features that could correlate with the paper's findings. I recommend reading Aiuppa et al. 2006 GRL and Tamburello et al. 2011 JVGR. These historical fumaroles should be plotted at least in Fig. 1 and 6.
- Authors describe the colour of the 4 different surfaces. I suggest that it could be more straightforward to show these colours (or a palette of colours for each type) in one of the figures;

- Please describe in Figure 4 caption what the letters a-g are;

- The bars in Figure 6a-b are hard to read. I suggest to use coloured circles with a colour bar. I suggest to calculate also the ratio between fluxes (CO2/S_tot, where S_tot = SO₂ + H₂S) and to plot their distribution to highlight the role of sulfuric gases
150 Fumarolic temperature rose up to 690 °C in May 1993 (Chiodini et al., 1995)
Chiodini G., Cioni R., Marini L. and Panichi C. (1995) Origin of fumarolic fluids of Vulcano Island, Italy and implications for volcanic surveillance. Bull. Volcanol. 57, 99–110. http://dx.doi.org/10.1007/BF00301400
240 Please explain how the 40°C threshold has been chosen;

530 Also, halogen may play a role in chemical leaching (Aiuppa et al., 2009 Chem Geo);

543 "higher gas flux" should be "higher acid gas fluxes"?

544 "we observe similar" looks incomplete

Citation: https://doi.org/10.5194/egusphere-2023-1692-RC2
- AC2:
  'Reply on RC2', Daniel Müller, 24 May 2024
  Dear Reviewer 2
  We thank you for reviewing and supporting our manuscript. Your review was highly appreciated and helpful to sharpen our manuscript. We carefully read through your suggestions and addressed all the points mentioned. We have changed the manuscript considerably and hope that it will now meet with your approval.
  In detail, we have modified:
  figure captions, which now better explain all necessary information shown in the figure
  
  the authorship by including publications and results of earlier works as suggested
  
  the discussion, now referring to results of earlier works and better highlighting how we contribute but also better synthesizing our results.
  
  We hope our replies are accepted and modifications made improve the manuscript. A more detailed reply to your revision is provided in the attached reply letter (pdf, You will find your original review text in black and our responses as reply in blue ) and as a text copy below.
  Thank you very much for supporting our work with your review and expertise
  Best Regards
  Daniel Müller (on behalf of all authors)
  
  Detailed replies to Reviewer 2
  Reviewer 2: The paper presents a comprehensive and meticulous investigation into the degassing and alteration structures of the fumarole field at La Fossa cone on Vulcano, offering valuable insights into the complex dynamics of volcanic activity. Using innovative methodologies, including close-range remote sensing, mineralogical and geochemical analyses, and surface degassing measurements, the authors provide a detailed and multi-faceted examination of the degassing system.
  One of the study's most commendable aspects is its integration of high-resolution drone-derived imagery with traditional analytical techniques. This approach enables a nuanced spatial analysis, allowing the authors to accurately identify and characterize major active units. Furthermore, the quantification of thermal energy release provides valuable quantitative data on the relative importance of different degassing features within the system, enhancing our understanding of volcanic processes.
  The authors demonstrate a commendable level of transparency and rigour by acknowledging potential limitations, such as gas plume distortion affecting image quality. This ensures the reliability and validity of their findings, reflecting a commitment to scientific integrity and strengthening the credibility of the study's conclusions.
  Overall, this paper represents a significant contribution to the field of volcanic degassing research. By elucidating the complex interplay between surface manifestations, alteration gradients, and gas emissions, the study advances our understanding of volcanic systems and provides a solid foundation for future research in this area. Its comprehensive approach and meticulous attention to detail make it a valuable resource for scientists and researchers working in volcano monitoring and hazard assessment.
  Reply: We appreciate this positive feedback and thank you for reviewing our manuscript. We addressed all your suggestions and modified our manuscript accordingly. Please find the detailed replies to each of your suggestions below.
  Reviewer 2: Here I list minor comments that I hope may improve the final version of the manuscript:
  The authors mention some fumaroles (e.g., F0) but do not show them in the figures in the main text (only supplementary). They are not also mentioned in the discussion. The fumaroles F0, FA, F5AT and F11 have distinct features that could correlate with the paper's findings. I recommend reading Aiuppa et al. 2006 GRL and Tamburello et al. 2011 JVGR. These historical fumaroles should be plotted at least in Fig. 1 and 6.
  Reply: We agree that showing the locations of fumaroles could help to better orient in the Figures and added the locations of fumaroles F0, F5AT, F11, and FA to Figure 1 and Figure 6, and the respective description in the Figure captions. Further, we added the labels for relative flux values to Figure 6A/B.
  Reviewer 2: Authors describe the colour of the 4 different surfaces. I suggest that it could be more straightforward to show these colours (or a palette of colours for each type) in one of the figures;
  Reply: We appreciate this comment. We intended to show color samples for surface types 1-4 in Figure 4 below Figures A and B as small subfigures. We understand that they were too small for easy viewing. We have now increased the size of these color samples and added descriptions to the figure caption.
  
  Reviewer 2: Please describe in Figure 4 caption what the letters a-g are;
  Reply: We agree and now better highlight in the Figure caption what the labels a-g mean. The respective sentence now reads “The labels a-g demark notable large-scale anomaly units that can be observed in both, the optical and the thermal data.”
  
  Reviewer 2: The bars in Figure 6a-b are hard to read. I suggest to use coloured circles with a colour bar. I suggest to calculate also the ratio between fluxes (CO2/Stot, where Stot = SO2 + H2S) and to plot their distribution to highlight the role of sulfuric gases
  Reply: We appreciate this comment. However, we tried different versions of this figure before but concluded that the bar plot was the best representation for showing the relative fluxes. Calculating the ratios from these measurements would certainly be interesting. However, we feel that calculating ratios from our results is not optimal due to the differential uncertainties of the instrument applied. We would feel a need to compare measured ratios to real in-situ data first. We have more gas data from other years and will look into that in more detail, but note that this is beyond the scope of the present study.
  Reviewer 2: 150 Fumarolic temperature rose up to 690 °C in May 1993 (Chiodini et al., 1995) Chiodini G., Cioni R., Marini L. and Panichi C. (1995) Origin of fumarolic fluids of Vulcano Island, Italy and implications for volcanic surveillance. Bull. Volcanol. 57, 99–110. http://dx.doi.org/10.1007/BF00301400
  Reply: We appreciate this comment and modified the text. It now reads: “Gases of the high-temperature fumaroles (HTF) emerge with temperatures >300 °C, but temperatures have been exceeded during previous volcanic crises (Harris et al., 2012, Diliberto, 2017). Temperatures of up to 690 °C were reported from May 1993 by Chiodini et al. (1995).”
  Reviewer 2: 240 Please explain how the 40°C threshold has been chosen;
  Reply: We appreciate this comment and have now defined thermal units and temperature thresholds in more detail.  Specifically, the temperature thresholds were defined after analyzing our infrared and optical data as well as based on previous knowledge of fumarole locations from previous field campaigns. When classifying pixels based on their temperatures, they form spatial clusters. We found that the 40°C temperature threshold outlines well the physical locations (depressions or fracture-like shapes in the surrounding fumarole crust) of major high-temperature fumaroles. The 22-40°C threshold defines larger contiguous clusters, which we interpret as rather diffuse features. However, these assumptions are partly arbitrary but a necessary approximation in order to define spatial boundaries and be able to quantify thermal emissions, size and extent of different active units.
  In the text, we changed lines 240 and the following to: “The temperature map was used to define the thermal structure. We observed several distinct thermal spatial units with temperatures significantly above the background temperature, that can be distinguished in high-temperature fumaroles (HTF in the following) and areas of rather diffuse thermal surface heating (Figure 4 B/D). To constrain these units spatially for further comparison, we had to approximate spatial boundaries what was done after comparison to our optical data and based on knowledge of previous observations by defining the temperature thresholds of T = 22-40°C for the diffuse heated areas and T> 40°C for HTF. The 40°C threshold resembles well the known locations and extent of HTF in the upper fumarole field.”
  Reviewer 2: 530 Also, halogen may play a role in chemical leaching (Aiuppa et al., 2009 Chem Geo);
  Reply: Yes we agree with this statement. We cannot constrain this process with our data set but have now added a reference to this possibility.
  Reviewer 2: 543 "higher gas flux" should be "higher acid gas fluxes"?
  Reply: We agree and modified the phrase to “… a higher acid gas flux…” as suggested.
  Reviewer 2: 544 "we observe similar" looks incomplete
  Reply: We appreciate this comment and changed the respective sentence to the following wording: “Analyzing the broader area of the central crater region we can infer multiple other areas where we observe similar changes of colorization indicating similar argillic or strong silicic alteration effects at the surface, in particular, located on the southern inner crater, the outer crater rims, the 1988 landslide area (Madonia et al. 2019) and the northern flank towards Vulcano Porto. These zones of strong alteration are indicated in red in Figure 1B.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1692-AC2

Daniel Müller, Thomas R. Walter, Valentin R. Troll, Jessica Stammeier, Andreas Karlsson, Erica De Paolo, Antonino Fabio Pisciotta, Martin Zimmer, and Benjamin De Jarnatt

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Short summary

We use drone-based optical and infrared remote sensing to analyze a volcanic degassing system. Anomaly detection allows us to reveal the degassing and alteration structure, which will be evaluated by mineralogical and geochemical analysis. A comparison of the defined anomaly pattern to present-day diffuse degassing and the thermal pattern allows to constrain major active units and to constrain their contribution to the total activity. We provide a detailed anatomy of a degassing system.


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